1. Technical Field
The present disclosure relates to a (poly(3,4-ethylenedioxythiophene:poly(styrenesulfonate) (hereinafter referred to as PEDOT:PSS)-based electrode and a fabrication method thereof. More particularly, the present disclosure relates to a PEDOT:PSS-based electrode having improved electrical conductivity, a method for fabricating the same, and an organic electronic device having the same.
2. Related Art
Flexible electronic devices are light and flexible, and thus are an aggregate of science and technology, which can be advantageously used in ubiquitous environments. One of the most important factors in the development of such electronic devices is the development of plastic transparent electrodes. Transparent electrodes are used in various applications, including flat panel displays such as LCDs, PDPs and OLEDs, touch screens, thin film solar cells, etc.
The most representative transparent electrode that is currently used is an indium tin oxide (ITO) electrode showing excellent optical and electrical properties. However, ITO is difficult to use in the field of next-generation flexible devices, due to its brittleness. Also, it requires high-temperature deposition, and for this reason, there has been a limit to fabricating high-performance transparent electrodes using ITO by printing processes. In addition, because the reserves of indium that is the main component of ITO are very small, the price thereof is rising. Thus, there is an urgent need for the development of new transparent electrodes.
As transparent electrode materials to substitute for ITO, carbon nanotubes, graphene, silver nanowires, metal oxides and the like have been much studied. However, plastic transparent electrodes developed to date have significantly low electrical conductivity, and thus there is an urgent need to overcome this shortcoming. As other transparent electrode materials to substitute for ITO, electrically conductive polymers comprising organic compounds have advantages including processability, lightweight, flexibility, a simple coating process, low production costs, etc., which are the advantages of general plastics, while these polymers are electrically conductive like metals, and show high transmittance in the visible region. Thus, these polymers are receiving attention as an alternative to ITO. In addition, conventional alternative materials comprising organic compounds necessarily require an expensive complex deposition process to form electrodes having an electrical conductivity comparable to that of ITO, whereas electrically conductive polymers have an advantage in that these can be formed into electrodes using a low-temperature solution process.
However, the relatively low electrical conductivity of electrically conductive polymers made it difficult to substitute these electrically conductive polymers for ITO.
PEDOT:PSS (poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), a kind of representative electrically conductive polymer, has high conductivity, and high transmittance in the visible region, and is soluble in water so that a solution process can be applied thereto in an environmentally friendly manner. In addition, it has high stability, and is one of electrically conductive plastic materials that are being most widely used (FIG. 1). However, it has an electrical conductivity of 1 S/cm, which is too low to use it as a transparent electrode and is significantly lower than that of ITO (>5,000 S/cm).
For past several decades, many studies on the optical and electrical properties of PEDOT:PSS have been conducted, and there have been attempts to improve the electrical conductivity using various organic solvents or surfactants or by acid treatment.
According to non-patent document 1 (Yijie Xia, Kuan Sun, and Jianyong Ouyang, Solution-Processed Metallic Conducting Polymer Films as Transparent Electrode of Optoelectronic Devices, Advanced Materials 2012, 24, 2436-2440) published recently, there is a report that an electrical conductivity of 3,065 S/cm was obtained by dropping a 1.0 M sulfuric acid (H2SO4) solution onto a PEDOT:PSS thin film. However, non-patent document 1 did not suggest a specific mechanism for improvement in the electrical conductivity of the thin film and an optimum fabrication method, and did not realize electrical properties comparable to those of ITO. Thus, the actual commercial use of the technology of non-patent document 1 is limited.
Accordingly, there is a need to develop a transparent electrode showing electrical properties comparable to those of ITO, by providing an optimum fabrication method that improves the electrical conductivity of electrically conductive polymers.